1. Field of the Invention
The present invention relates to a process for processing OFDM-signals received simultaneously by a multi-antenna system with a plurality of separate receiving channels.
2. Description of the Background Art
In modern digital technology, so-called OFDM-systems (Orthogonal-Frequency-Division Multiplex) or COFDM-systems (coded OFDM) are used for data transmission (sound, video or other data). In accordance with this principle, prior to transmission the digital data stream is split via a transmitter network into a plurality of sub-signals, each of which is transmitted separately on an individual carrier. In the so-called DVB-T-system (Digital-Video Broadcasting, terrestrial), which also serves for the transmission of data of a general type, 1705 or 6817 individual carriers are used for example. In the receiver these items of subsidiary information are recombined to form a complete item of information of the transmitter-end digital data stream.
These OFDM-systems are standardized in terms of the transmitting-end conditioning and receiving-end recovery of the data (for example in the DAB-standard ETS 300401 for DAB and in the standard ETS 300744 for DVB-T). It is a common feature of these OFDM-systems that at the receiving end the high-frequency signal received by an antenna is demodulated in an OFDM-demodulator, preferably after conversion into an intermediate frequency, and in this way the associated I/Q-values are acquired for each individual carrier. In a so-called pilot-tone-corrected OFDM-system, as used in DVB-T, a channel correction value is determined simultaneously from the co-transmitted pilot tones. For each individual carrier, each I/Q-value is complexly multiplied by the relevant channel correction value. This ensures that all the carriers have constant amplitudes, possible breaks in amplitude of individual carriers of the overall reception band, caused for example by multipath reception disturbances, being appropriately compensated and corrected.
In such systems, in addition to the individual data, it is also common practice to transmit so-called confidence values and thus to influence the further processing of the acquired digital values in so-called soft-decision processes. These two known possibilities of correcting the I/Q-values via the channel correction or the obtained digital values through the confidence values are state of the art in receiver technology.
To improve the signal/noise ratio, in particular for the mobile reception of such OFDM-signals, it is known to provide a multi-antenna system with two or more antennae and correspondingly assigned, separate receiving channels, and to combine the analogue received signals in the receiver in the HF- or IF-plane of this plurality of receiving channels. The analogue signals of the individual receiving channels are added, having been weighted in a frequency-dependent manner, for example, as a function of the received power. Here, however, not only the useful signals but also the noise components are combined, which in principle can even result in an impairment of the signal/noise ratio compared to the most favourable receiving channel for the relevant sub-band. These analogue combining processes also require a very high outlay and follow the relevant channel properties only relatively slowly. In the case of frequency-selective addition, they have only relatively flat selection curves, i.e. sharp breaks in the receiving frequency range cannot be corrected.